Most BL Lacs are believed to be low-luminosity Fanaroff-Riley class I radio galaxies (FR-1s) whose jet axes are oriented towards the observer such that their emission is dominated by relativistically-boosted synchrotron and inverse-Compton emission, making them ideal laboratories for understanding the physics of beaming and relativistic jet propogation. Currently there are only two statistically-complete samples of BL Lacs: the radio-selected 1Jy sample (RBLs) and the X-ray-selected EMSS sample (XBLs). While once extremely rare, new X-ray and radio surveys are increasing the number of known BL Lacs dramatically, creating an impetus to understand their nature. It has been suggested that RBLs and XBLs are both beamed FR-1s but that XBLs are either viewed farther from the jet axis or are lower in intrinsic luminosity than RBLs. However these hypotheses are difficult to test because the highly-beamed emission is so dominant that any unbeamed emission is difficult or impossible to detect; as such many BL Lacs have lacked redshifts or detected extended radio emission for some time. However, modern instrumentation and data reduction techniques have finally allowed us to obtain redshift information for all but four objects in the 1Jy sample; and high dynamic-range (10,000:1) VLA mapping has finally detected extended emission around all but three of the highly core-dominated 1Jy sources. Multi-wavelength observations, including X-ray, optical, near-IR and radio (VLA and VLBA) imaging and spectroscopy, for both the EMSS XBL and 1Jy RBL samples are now complete; and these observations will be presented. While many of the differences between XBLs and RBLs are consistent with the beaming hypothesis, differences in the spatial distributions and physical properties (e.g., emission-line and extended-radio luminosities) of the two samples suggest a more complex picture, wherein RBLs can also be caused by either beamed high-luminosity FR-2s or by gravitationally-lensed quasars. The implications of these observations for unification models will be discussed.
American Astronomical Society Meeting Abstracts
- Pub Date:
- December 1997